The present invention is strain-crystallizing poly{methyl(3,3,3-trifluoropropyl)siloxane} elastomers and methods for their making. The present process comprises the cross-linking of cis stereoregular polymers prepared by reacting isomers of 1,3,5-trimethyl-l,3,5-tris(3',3',3'-trifluoropropyl)cyclotrisiloxane, where at least 30 percent of the isomers are of cis configuration, in the presence of an aprotic polar promoter and a lithium compound. The lithium compound initiates ring opening and polymerization of the isomers without affecting the steric arrangement of the isomers. Elastomers prepared by cross-linking these stereoregular polymers have sufficient cis 3,3,3-trifluoropropyl substitution to allow the elastomers to strain crystallize and provide reinforcement to the elastomers at typical use temperatures.
Among the general requirements for strain induced crystallization in elastomers are a base polymer with a low glass transition and additionally a melting transition within about 5.degree. C. to 20.degree. C. of use temperature. Typically, strain-induced crystallization in silicon elastomers is not observed because they either show no melting transition or their melting points are so low that the phenomena is precluded at use temperatures.
Described herein are poly{methyl(3,3,3-trifluoropropyl)siloxane polymers having a sufficient degree of stereoregularity such that when they are cross-linked to form an elastomer, the elastomer has a melting transition which allows strain induced crystallization of the elastomer under typical use conditions.
It is known in organosilicon art that cyclic diorganosiloxanes can be polymerized to high polymers by heating them with alkaline catalysts, such as potassium hydroxide or its corresponding siloxane salts. This has become a predominant method for the production of siloxane elastomers. However, during this alkaline polymerization, breaking of the siloxane ring to form high polymers and degradation of high polymers to form cyclics is occurring constantly. Since these polymerization and degradation reactions occur at different rates, the resulting product represents an equilibrium between the two processes. Because of these competing reactions, any polymer which is ultimately formed, by these processes will be atactic lacking significant stereoregularity.
Bostick, U.S. Pat. No. 3,337,497, issued Aug. 22, 1969, describes a process for polymerizing cyclotrisiloxanes that does not result in the equilibrium processes described above. Specifically, Bostick teaches that ordered copolymers can be formed by reacting a mixture comprising a first cyclotrisiloxane, an aprotic solvent, and an organolithium compound and thereafter adding a second cyclotrisiloxane to the process and reacting. Bostick teaches the stereo-specific opening of cis 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane when reacted in tetrahydrofuran with n-butyl lithium. Bostick further teaches that the presence of halogens, particularly, fluorine on the aliphatic carbon attached to silicon markedly increases the randomness of the formed polymers and should be avoided in the cyclic polysiloxanes.
Lee, U.S. Pat. No. 3,575,921, issued Apr. 20, 1971, describes the reaction of cis 1,3,5-triphenyl-1,3,5tris(3',3',3'-trifluoropropyl)cyclotrisiloxane in the presence of sec-butyl lithium. The product is reported to be a stereoregular material in which one of the cis 3,3,3-trifluoropropyl groups of each trimer is shifted to a trans position.
Curtis et al., Polym. Preprin., Div. Polym. Chem., Am. Chem. Soc., 25 (1984) 1, describe the reaction of cis 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane in the presence of lithium trimethylsilanolate. The product is reported to be formed by a head-to-tail insertion of the trimer.
Momper et al., Polym. Commun. 31 (1990) 186, teach the polymerization of cis and of trans 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane in the presence of hexamethylphosphoric acid triamide as a promoter and n-butyl lithium as an initiator. Momper et al. concluded that the products obtained through polymerization of the different isomers provided for different tacticities. Momper et al. further noted that some of the products exhibited a finite degree of crystallinity.
The discussed art does not teach the reaction of specific isomers of 1,3,5-trimethyl-1,3,5-tris(3',3',3'-trifluoropropyl)cyclotrisiloxane to prepare polymers with enhanced stereoregularity. Furthermore, the art does not teach that such polymers when cross-linked can have sufficient stereoregularity to allow the elastomer to strain crystallize. Bostick, supra, specifically teaches that the presence of fluorine on the aliphatic carbon attached to silicon markedly increases the randomness of the formed polymers and should be avoided in the cyclic polysiloxanes. Lee, supra, teaches a shift of one of the cis 3,3,3-trifluoropropyl groups to a trans position during reaction in the presence of a lithium compound. Therefore, unexpectedly the present inventors have discovered that 1,3,5-trimethyl-1,3,5-tris(3',3',3'-trifluoropropyl)cyclotrisiloxane can be reacted in the presence of an aprotic polar promoter and a lithium compound to form polymers having cis stereoregularity sufficient to allow elastomers fabricated from the polymers to strain-crystallize.
Polymers prepared by the process described herein can be cross-linked to form elastomers having high elongations to break along with high strengths, without reinforcing fillers. These elastomers can resemble the behavior of natural rubber and should have parallel applications, but with the added advantages that they have an inherent resistance to hydrocarbon solvents as well as to high temperatures and oxidation. The elastomers can be fabricated as room temperature vulcanizing compositions or molded in thermal cure processes. The elastomers may be used, for example, to form gaskets, O-rings, and diaphragms and as sealants and adhesives.